This invention relates to electromagnetic energy, and more particularly, to electromagnetic exposure of planar materials.
In microwave heating and drying applications involving waveguide structures, uniform heating is desirable but is only achievable if the ability exists to expose every section of the material (the web) to the same electric field intensity. Lossy materials absorb energy and thus cause attenuation of the electric field intensity in the dimension of propagation in the waveguide. As a result, the traditional technique of inserting the lossy materials longitudinally in the center of the waveguide results in a non-uniform distribution of energy across the width of the lossy material. To correct this, it is necessary to manipulate the electric field distribution in the waveguide such that when a lossy material is placed inside, the effect due to attenuation is balanced by the initial electric field distribution. The net result is an electric field with the same intensity at all points along the material. This leads to the expression of xe2x80x9ccompensating for the attenuation.xe2x80x9d
There are several proposed methods for compensating for attenuation. One method is to insert the web into a diagonal slotted waveguide structure as is described and claimed in U.S. Pat. No. 5,958,275, which is incorporated by reference in its entirety. In essence, this method achieves uniformity by physically changing the material""s position within the electric field distribution. This is very effective for uniformly exposing thin materials to microwave energy over a wide web. Unfortunately, for thicker dielectric materials within a diagonal slotted waveguide, uniformity is more difficult to achieve, due to the xe2x80x9cskewingxe2x80x9d of the electric field by the material. Unlike a thin material, the thicker material cannot be inserted into the guide without it having a significant effect on the electric field distribution.
A device for heating a material comprises a rectangular waveguide with an elongated opening for passing a planar material through the rectangular waveguide. A source creates an electric field between a top surface and a bottom surface of the rectangular waveguide. The electric field is controlled to compensate for attenuation of the electric field. The electric field can be controlled by, for example, using a dielectric slab along the top surface of the rectangular waveguide or a tapered dielectric slab along the top surface of the rectangular waveguide. The electric field can also be controlled by, for example, making the waveguide appear electrically wider at one end. The waveguide can be made to appear electrically wider at one end by, for example, inserting one or more tapered fins. The tapered fins can be adjusted or remove to account for the lossiness of the planar material.
One advantage of the disclosed invention is that it is possible to heat thick, high-dielectric materials. Another advantage is that a tapered dielectric slab greatly simplifies the fabrication process and adds more flexibility to the overall system. Machining a dielectric slab with a specified taper is a relatively easy task. Instead of designing a different waveguide slot angle for each different material, the slot in the waveguide can now be the same for all materials, and different control slabs can be used for materials which need different tapers.